It is known to provide cooking utensils, especially baking dishes, trays, forms and the like with various coatings for different purposes, e.g. as non-stick coatings, as heat absorbers or transmitters, and as thermal conductors.
For the purposes of this discussion of the background of the invention and for the subsequent discussion of the invention itself, the term "baking form" will be used to describe any utensils having a recess in which a comestible may be baked. Typical of such "baking forms" are pie pans, cake pans, ring pans and the like.
The term "aluminum" as used herein to refer to baking forms and like utensils and as it will be used hereinafter in connection with the discussion of the invention, is intended to include all conventional aluminum alloys which have been hitherto used for the production of baking utensils and existing alloys suitable for use in baking utensils whether or not they have been so used heretofore.
Baking utensils, especially baking forms of the type described, are generally fabricated from sheet aluminum by deep drawing. The sheet material has also been constituted of aluminum sheet or strip heretofore. The only requisite has been that the metal from which the baking form was drawn have deep-drawing qualities.
As long as the baking form is constituted of steel sheet, it is necessary to provide corrosion protection against rusting of the inner and outer surfaces.
To this end, anticorrosion coatings are applied internally and externally and it has been found that tin coating (to produce tin plate) or, more recently, coatings of polytetrafluoroethylene are satisfactory for this purpose.
Coatings of polytetrafluoroethylene, however, suffer from poor adhesion to the substrate and reduce the heat transfer rather than improve the same. As a result, it is difficult to brown the comestibles which are subject to baking. Browning thus either never occurs or is effected only at high baking temperatures which may be detrimental to the coating.
The heat transfer is primarily by heat conduction and experience has shown that infrared radiation is scarcely capable of being passed through the walls of the baking form.
The "browning" of the baked material is, however, effected principally by the infrared radiation.
The foregoing applies to baking forms which consist of aluminum sheet provided internally and externally with colorless aluminum-oxide coatings and/or with polytetrafluoroethylene.
It has been found to be possible, in prior work in this field, to improve the heat transfer of the baking forms to the comestible material to be baked by forming the baking utensil from copper sheet. In fact, baking utensils have also been formed from steel sheet provided with a brownish coating. Neither technique has been fully successful because of the problem of corrosion. The latter techniques have never influenced the aluminum-sheet technology in dealing with aluminum baking utensils and, as far as I am aware, there has been no fully successful attempt to improve the heat transfer of aluminum-sheet baking utensils to the comestible product.